© 2000 Scott S Albert Structured Programming 256 Chapter 7 Streams and File I/O

© 2000 Scott S Albert

Structured Programming 256Chapter 7

Streams and File I/O


I/O Overview

• I/O = Input/Output

• In this context it is input to and output from programs

• Input can be from keyboard or a file

• Output can be to display (screen) or a file

• Advantages of file I/O

– permanent copy

– output from one program can be input to another

– input can be automated (rather than entered manually)


Streams

• Stream: an object that either delivers data to its destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.)

– it acts as a buffer between the data source and destination

• Input stream: a stream that provides input to a program

• Output stream: a stream that accepts output from a program– cout is an output stream– cin is an input stream

• A stream connects a program to an I/O object– cout connects a program to the screen– cin connects a program to the keyboard


Binary versus text files• All data and programs are ultimately just zeros and ones

– each digit can have one of two values, hence binary

– bit is one binary digit

– byte is a group of eight bits

• Text files: the bits represent printable characters

– one byte per character for ASCII, the most common code

– for example, C++ source files are text files

– so is any file created with a "text editor"

• Binary files: the bits represent other types of encoded information, such as executable instructions or numeric data

– these files are easily read by the computer but not humans

– they are not "printable" files

• actually, you can print them, but they will be unintelligible

• "printable" means "easily readable by humans when printed"


Opening a new output file

• The file name is given as a String– file name rules are determined by your operating system

• Opening an output file takes two steps

1. Create a ofstream object associated with the file name String

# include <fstream.h>

2. Use the open method to connect the file name


Example: opening an output file

To open a file named numbers.dat:

ofstream out_file;

out_file.open(“numbers.dat”);

To write to the file,

Use out_file<<


Every file has two names

• The code to open the file creates two names for an output file

– the name used by the operating system• numbers.dat in the example

– the stream name• out_file in the example

• C++ programs use the stream name– out_file in the example


Closing a file

• An Output file should be closed when you are done writing to it

• Use the close method of the class ofstream

• For example, to close the file opened in the previous example:

out_file.close();

• If a program ends normally it will close any files that are open


If it is done automatically,why explicitly close files?

If a program automatically closes files when it ends normally, why close them with explicit calls to close?

Two reasons:

1. To make sure it is closed if a program ends abnormally (it could get damaged if it is left open).

2. A file open for writing must be closed before it can be opened for reading.

• Although C++ does have a class that opens a file for both reading and writing, we will not discuss this topic


Warning: overwriting a file

• Opening a file creates an empty file

• Opening a file creates a new file if it does not already exist

• Opening a file that already exists eliminates the old file and creates a new, empty one

– data in the original file is lost

• How to test for the existence of a file and avoid overwriting it will be covered later


Opening a new input file

• Similar to opening an output file, but replace "output" with "input"

• The file name is given as a String– file name rules are determined by your operating system

• Opening a file takes two steps

1. Creating a ifstream object associated with the file name String

2. Connecting the ifstream to a file name


Example: opening an input file

To open a file named numbers.dat:

ifstream in_file;

infile.open(“numbers.dat”);

To read from the file,

Use in_file>>


Input file exceptions

• If (in_file.fail()) cout<<“Error opening file\n”;

in>>x;while(!in.eof()){

cout<<x<<endl;in>>x;

}

Example:reading a file name from the

keyboard

import java.io.*;

public class FileNameDemo{ public static void main(String[] args) throws IOException { System.out.println("Enter file name:"); String fileName = SavitchIn.readLineWord(); DataInputStream inputStream = new DataInputStream(new FileInputStream(fileName));

System.out.println("Reading and summing the nonnegative"); System.out.println("integers in the file " + fileName); int sum = 0; int n = inputStream.readInt(); while (n > 0) { sum = sum + n; System.out.println(n); n = inputStream.readInt(); } System.out.println("End of reading from file."); inputStream.close(); System.out.println("The sum of the numbers is " + sum); }}

FileNameDemo

reading a file name from the keyboard

closing the file

using the file name read from the keyboard

reading data from the file


Common methodsto test for the end of an input file

• A common programming situation is to read data from an input file but not know how much data the file contains

• In these situations you need to check for the end of the file

• There are two common ways to test for the end of a file:

1. Put a sentinel value at the end of the file and test for it.

2. Test for a special character that signals the end of the file (text files often have such a character).


Summary• Text files contain strings of printable characters; they look intelligible to

humans when opened in a text editor.

• Binary files contain numbers or data in non-printable codes; they look unintelligible to humans when opened in a text editor.

• C++ can process both binary and text files

• Always check for the end of the file when reading from a file. The way you check for end-of-file depends on the method you use to read from the file.

• A file name can be read from the keyboard into a String variable and the variable used in place of a file name.

• The class fstream has methods to test if a file exists and if it is read- and/or write-enabled.

Documents

© 2000 Scott S Albert Structured Programming 256 Chapter 7 Streams and File I/O